Method of pressure equalizing for blast furnace top



United States Patent Inventor 3,276,757 10/1966 Owen 3,343,825 9/1967 Tsutsumietal ABSTRACT: A blast furnace top structure, having the usual charging bells whereby material may be charged into the furnace without the loss of gas pressure in the furnace top, in which means are provided for discharging gas and steam simultaneously into the space between the bells and for controlling said discharge whereby the bell space pressure may quickly be elevated from substantially atmospheric to or above the furnace top pressure. The apparatus is adapted to carry out a method for equalizing the pressure in the bell space with the furnace top pressure by use of gas from an external source (for example, cleaned furnace gas) at a pressure lower than the furnace top pressure in which steam is added to the gas whereby the pressure of the air, gas and steam in the bell space is rapidly elevated to the desired degree at/or above the furnace top pressure.

METHOD OF PRESSURE EQUALIZING FOR BLAST FURNACE TOP BACKGROUND OF THE INVENTION Present day practice inblast furnace design is to provide two or more movable bells in the top structure of the furnace. The space between the bells provides chamber or chambers which can be alternately opened to the furnace top or to the atmosphere. In this way, the furnace can be charged with materials without loss of the gas pressure in the furnace top. Blast furnaces are operated with furnace top gas pressure maintained as high as possible, currently up to 25 pounds per square inch gauge, to improve the efficiencyof the reducing operation in the furnace.

For purposes of simplifying the presentation of this invention, the description will refer to a two bell furnace. However, furnaces with more than two bells also require equalization of pressure in bell spaces, and the invention applies to both two bell furnaces, and to those with more than two bells.

When introducing charge material into a furnace, it is necessary to raise the pressure in the space above the lower large bell, in order to allow the bell to open against the furnace top pressure. Also, when both bells are closed, pressure in the bell space should be maintained at, or slightly higher than, furnace top presure to minimize gas flow through the space between the bell and the seat..At other periods of the charging cycle, the pressure must be reduced to atmospheric so that the small-bell may be opened and the furnace charge material deposited on top of the large bell. The process of altering the gas'pressure in the bell space is known as equalizing or balancing" although the pressure in the large bell space and in the furnace top are not necessarily held exactly equal. The most desirable condition is for the pressure in the bell space to be slightly higher than the furnace top pressure, but maintained as near to top pressure as possible. This prevents flow of dirty gas, with its abrasive properties, from the furnace top through any spaces between the large bell and its seat, such spaces being due to wear, imperfections in manufacture, warping of material due to temperature changes, etc. 7

Various gases are in use, or have been used or proposed for accomplishing pressure equalization. These include dirty gas directly from the furnace top; blast furnace gas which has been cleaned after leaving the furnace; gases produced in separate generators; or gases such as nitrogen purchased as a byproduct from an adjacent operation. It has also been proposed to use steam alone, without gas,'for pressure equaliz- Ihe importance of clean gas for equalizing is well known in the art, both to prevent scoring and erosion on bells, valves, etc., and also to reduce atmospheric pollution when the gas is discharged to atmosphere. For this reason, dirty gas directly from the furnace top is unsatisfactory.

- Clean blast furnace gas is readily available, but not at the requisite pressure. For example, present practice calls for blast furnace gas to pass through a dust catcher, then through scrubbing devices which clean the gas. However, gas scrubbers produce considerable drop in pressure in the gas passing through them. Modern scrubber designs frequently call for 80 inches of water column pressure drop, which is the equivalent of 3.45 pounds per square inch. Furthennore, to produce the cleanliness called for by present day standards, two scrubbers in series are usually required, thus producing twice the pressure drop of a single scrubber. Piping and control valves produce further pressure drop. Consequently, clean blast furnace gas is only available for pressurizing at a pressure which I is to 10 pounds per square inch lower than the furnace top pressure.

Similarly, separately generated gases are available only at pressures considerably below those required for balancing modern furnaces, unless some means is provided to compress the gas after generation.

LII

The use of a compressor driven by an electric motor has been proposed to increase the pressure of the equalizing gas. One such method would require the compressor to discharge directly into the bell space. as the time available for the pressurizing is usually less than 10 seconds, such a compressor would have to be very large, costly, and would require a large horsepower to drive it. Furthermore, such a compressor is too large to start and stop in the available time; therefore some arrangement would have to be provided to enable it to run continuously. i

To overcome the limitations of a compressor discharging directly into the bell space, another method has been proposed. This method would have the compressor discharge into an accumulator tank, which in turn would be connected to the bell space. When pressurizing of the bell space is called for a valve between the bell space and the accumulator would open and the gas would flow from the accumulator to the bell space. The limitations of this system are twofold available time and operating pressure. Available time for recharging the accumulator is less than seconds. Pressure in the accumulator must be raised to a value considerably above the furnace top pressure. The capacity of the accumulator depends both on the volume of the accumulator, and the difference in pressure in the accumulator at the start and when pressurizing is complete. After pressurizing is complete, the pressure in the accumulator must still be at least equal to the pressure in the bell space. Therefore, at the start of pressurizing, the pressure in the accumulator must be considerably higher than the pressure required in the bell space. Even with a large accumulator tank, the time available and the pressure required would require an uneconomically large compressor.

The use of steam alone as a substitute for equalizing gas is also unsatisfactory for maintaining the high top pressures desired in modern furnace operation. In such an application, the pressure of the steam in the bell space would require that the temperature in the bell space be maintained at a correspondingly high level to prevent the steam from condensing. Any leakage which occurs during the period when bell space is pressurized must be made up by steam only, which increases the proportion of steam in the bell space, making the condensation problem worse. Also, when the bell space pressure has been reduced to atmospheric prior to opening small bell, the bell space still contains a considerable amount of vapor. When the small bell is opened, the load of material discharging into the large bell space will cause the vapor to condense rapidly, which temporarily produces a pressure considerably below atmospheric in the bell space. This will interfere with the proper distribution of charge material in the bell space, as well as introducing structural hazards to the blast furnace structure and appurtenances.

SUMMARY OF THE INVENTION The present invention consists in the introduction of a combination of gas and steam into the bell chamber of a blast furnace, to quickly raise the pressure in the bell space to the most desirable level, and to maintain that pressure during the parts of the charging cycle which require such pressure. The gas may be clean blast furnace gas, nitrogen, or any other suitable gas or combination of gases. The gas may be at any pressure. but the invention will be particularly useful if the pressure of the gas supply is lower than the desired balancing pressure.

The steam accomplishes three purposes: (1) it increases the pressure of the balancing gas used by increasing the gas temperature; (2) it increases the pressure of the air in the bell space by increasing the air temperature; (3) it provides any additional pressure needed to make up the total balancing pressure desired.

This invention eliminates the need for compressors and accumulator tanks. With this invention, the pressure in the bell space can be produced quickly and maintained closely. It avoids condensation problems by using the available pressure of other gases, and operating at a lower vapor temperature and pressure than would be required if steam alone were used.

It will work equally well with blast furnace gas, or other suitable available gases.

BRIEF DESCRIPTION OF THE DRAWINGS .clean gas in accordance with the method disclosed herein I FIG. 3 is an illustrative chart showing the bell space temperature, after pressure equalizing according to the method disclosed herein, when clean gas is supplied at pressures from to 25 pounds per square inch gauge.

DESCRIPTION OF THE ILLUSTRATED EMBODIMENT I FIG. 1 illustrates the top of a two bell blast furnace, generally indicated at F, the upper space 1 within the furnace being, of course, under the furnace top-pressure which, as

' previously noted, under present blast furnace practice may be as high as 25 pounds per square inch gauge. The charging hopper H is provided with the usual lower large bell 2 and upper small bell 3 which together with the hopper walls define the bell space 4.

To introduce a charge into the furnace F, the large and small bells 2 and 3 being closed as seen in FIG. 1, the charge is first disposed in the upper part of hopper H on top of the closed small bell 3. The pressure within the bell space 4 is then reduced substantially to atmospheric by opening the bypass valve 15b which vents the bell space 4 to atmosphere and the small bell 3 lowered to permit the charge to drop into the bell space 4 on top of the closed large bell 2. The valve 15b is closed and small bell 3 is then lifted into closed position and the pressure in bell space 4 is balanced with pressure in the furnace F. The cycle just described whereby charge is disposed in upper part of hopper H, and this charge material is transferred to bell space 4, may be repeated once or several times. Before each opening of the small bell 3 the pressure in the bell space 4 is reduced to atmospheric, and after the small bell'is closed, pressure in the bell space 4 is raised to balance the pressure in the furnace F, as will be later described. When a suitable amount of charge material has accumulated in bell space 4, and the small bell 3 has been lifted into closed posi- I pipe 5 and a pipe 6 leads from a source of supply of clean gas,

preferably the clean blast furnace gas main from the gas cleaners. As previously noted, clean blast furnace gas in the pipe 6 will necessarily be at a pressure lower than the furnace top pressure because of the pressure drop in the dust catchers and gas scrubbers.

' A gas inductor 7, consisting of a housing 7a, a steam nozzle 7b and .a blending nozzle 70, is connected to the end of the clean gas pipe 6. The steam nozzle 7b of the inductor 7 is at the end of the steam supply pipe 8 which is connected to any suitable source of steam at the desired temperature and at a pressure which is higher than the desired balancing pressure which is to be established within the bell space 4. The steam pressure regulating valve 9 in the steam supply line 8 is set to maintain the desired constant steam pressure at the steam control valve 10 which controls the flow of steam to the inductor 7. A steam and gas mixture supply pipe 11 leads from the blending male 70 to the bell space 4. A check valve 12 is mounted in the gas supply pipe 6 to prevent back flow into the clean-gas supply when the inductor 7 is not operating and a relief valve 15 in the pipe 15a which leads from the bell space 4 to atmosphere may be set to prevent excessively high pressure in the bell space 4 by relieving such space 4 by relieving such space to atmosphere.

The pressure relating valve 10, which controls the flow of steam to the inductor 7 is responsive to the differential pres.-

sure between the furnace top space 1 and the bell space 4 and may be of any suitable and well-known type whereby, if the pressure in the bell space 4 falls below a predetermined point (equal to or slightly greater than the furnace top pressure), valve 10 will be opened until the desired bell space pressure is established. Of course, if the bell space pressure becomes too high the reverse-operation will occur and, by the proper setting of the operating unit 10' of the pressure regulating valve 10, the desired bell space pressure relative to the furnace top pressure will automatically be established and maintained.

The operation of the apparatus shown in FIG. 1 is as folv lows: when it is desired to balance the pressure in the bell space 4 with the pressure in the furnace top space 1 the bells 2 I and 3 are closed. Steam is then allowed to enter through the chamber and in the mixing nozzle 70, after which the mixture I passes through the pipe 11 into the bell space 4.

The time in the furnace charging cycle when it is desired to raise the bell space pressure to or slightly above the furnace top pressure is just after the charge has been dropped into the bell space 4 by lowering the upper bell 3. During the dropping of the charge into the bell space the supply of steam has been shut off by closing the valve 8a in pipe 8. The upper bell 3 is closed and the valve is now opened permitting steam to pass through the inductor 7, mixing with clean gas, and the mixture then discharged into the bell space 4. The hot steam will heat the air in the bell space 4 and the introduction of steam and gas will continue until the desired pressure is reached in the bell space 4. The flowof steam to the inductor 7 is controlled through the valve operating unit 10' in accordance with the pressure differential between the furnace top 1 and the bell space 4 through the pipe connections 13 and 14. After the desired pressure is reached in the bell space 4 further flow of steam and gas will be shut off by the valve 10 except for such flow as is required to make up for any leakage from the bell space 4. The controller 10' acts on the valve 10 in accordance with the pressure differential between the furnace top 1 and the bell space 4 to effect such pressure maintenance. The atmosphere relief valve 15 is set to relieve any excessive pressure which may possibly develop in the bell space 4 and the check valve 12 in the gas supply line 6 prevents back flow from the bell space into the gas supply line.

The particular arrangement shown is only one example of how this invention can be applied to a blast furnace. Alternative arrangements and modifications will be apparent to those skilled in the art. The steam and gas can be introduced separately into bell space, with separate controls on each. Overall control of final pressure can be effected by sensing pressure in bell space only. The control can be automatic .or manual.

In order to demonstrate the operation of the method, and.to illustrate its advantages, the performance for certain specific conditions is shown in FIG. 2 and 3. The examples are intended to illustrate the principles of the invention and are not intended to limit the application of the invention in any way.

In the following explanation p.s.i." signifies pounds per square inch, p.s.i.a. signifies pounds per square inch absolute, and p.s.ig. signifies pounds per square inch gauge which is equal to p.s.i.a. minus the assumed atmospheric pressure of l4.7 p.s.i.a.

The charts shown in FIGS. 2 and 3 are based on the following assumptions:

Atmospheric pressure 14.7 p.s.i.a.

Gauge pressure Absolute pressure minus 14.7 p.s.i.

Desired final pressure in bell space 25' p.s.i.g. or its equivalent, 39.7 p.s.i.a.

Balancing gas is assumed to be clean blast furnace gas having approximately the same specific heat and gas constant as air, and having a specific gravity of 1.02 as compared with air. Balancing gas assumed to be at 100 F. at source. Air in bell space is assumed to be at 50 F. before pressurizing starts.

Volume of bell space 7,000 cubic feet.

FIG. 2 shows the pressure in the bell space in p.s.i.a. plotted against available pressure of clean gas in p.s.i.g. Reading from the chart, when clean gas at p.s.i.g. is used for pressurizing, the final pressure in the bell space, after pressurizing is completed in accordance with the above described procedure, is the sum of the following partial pressures:

It should be noted that both the air pressure and the clean gas pressure have been increased by heat from the steam.

FIG. 3 shows the final temperature in bell space, plotted against the available pressure of clean gas. This chart shows that for clean gas at an available pressure of 15 p.s.i.g., the final temperature in bell space after pressurizing is 158 F.

The charts also enable a comparison to be made between the results achieved with the present invention as compared with the use of steam only as a pressurizing medium. The quantities for steam only can be read from the charts, using the plotting for an available gas pressure of 0 p.s.i.g. Thus:

STEAM ONLY Bellspace Bellspace at pressurized atmospheric pressure Air pressure, p.s.i.a l9. 9 7. 4 Steam pressure, p.s.La 19. 8 7. 3

Total pressure, p.s.l.a 39. 7 14. 7

Temperature, "F 230 210 GAS AND STEAM (ASSUMING GAS AT 15 p.s.i.g.)

Bell space Bell space at pressurized atmospheric pressure Air presure, p.s.i.a 17. 9 6. 6 Gas pressure, p.s.i.a. 17. 0 6. 3 Steam pressure, p.s.i.a.. 4. 8 1. 8

Total pressure, p.s.i.a 39. 7 14. 7

Temperature, "F 158 150 gas and steam the pressure would be 1.8 p.s.i.a. To discharge cold stock into a bell space in which half of the atmosphere is steam would produce rapid condensation and lowering the pressure to a point which would be dangerous for the furnace structure, as well as disturbance to the distribution of stock.

It is to be understand that this invention is not to be limited to the specific descriptions and illustrations given herein and that it is to be given its broadest interpretation within the terms of the appended claims.

lclaim:

1. In a blast furnace having a bell chamber including independently movable upper and lower bells, said bells together with the walls of said bell-chamber defining a bell space and said furnace being adapted to operate at a predetermined top pressure, a source of supply of clean gas at a pressure less than said furnace top pressure, a, source of supply of steam at a pressure greater than said top pressure, means for discharging gas and steam from their respective sources simultaneously into said bell space, and means for controlling said introduction of gas and steam whereby the pressure in said bell space, with said bells closed, is raised to or above said furnace top pressure.

2. A blast furnace as described in claim 1 which includes means for mixing, outside of said bell space, gas and steam from their said sources of supply, and means for introducing the resulting mixture into said bell space, and in which said means for controlling the introduction of gas and steam includes a control valve for regulating the steam in said mixture response to the pressure differential between said furnace top and said bell space whereby the total pressure in said bell space is raised to a predetermined value at or above said furnace top pressure.

3. Apparatus for equalizing the pressure in the bell space and furnace top of a blast furnace including, a source of steam at a pressure higher than the furnace top operating pressure,

means for conducting steam from said source into said bell space, a source of gas at a pressure higher than atmospheric but lower than said furnace top operating pressure, means for conducting gas from said source of gas into said bell space, and means for controlling the flow of steam into said bell space responsive to pressure conditions within said bell space.

4. Apparatus for equalizing the pressure in a blast furnace as defined in claim 3 including means for mixing steam and gas from their respective sources externally of said bell space and means for conducting the resulting mixture into said bell space.

5. A method of equalizing the furnace top pressure of a blast furnace or the like having a bell space adjacent said furnace top, and the bell space pressure thereof comprising introduc' ing steam and an equalizing gas into said bell space when said space is closed and the pressure therein is substantially atmospheric, and controlling the flow of said steam in response to the pressure differential between said furnace top and said bell space whereby said bell space pressure is raised to a predetermined value at least equal to said furnace top pressure.

6. A method as defined in claim 5 in which said predetermined pressure in said bell space is maintained until said space is opened to said furnace top to permit flow of a charge into said furnace.

7. A method as defined in claim 5 in which said steam is at a pressure higher than the furnace top pressure and said gas is at a pressure lower than said furnace top pressure.

8. A method as defined in claim 5 in which the steam and gas are mixed externally of the furnace bell space before being introduced thereinto.

9. A method as defined in claim 7 in which said gas is cleaned blast furnace gas. 

